One of the themes that shows up a lot in Terry Brooks's Shannara series is the abuse of magic. The protagonists usually have to deal with antagonists that have abused magic in some form or have even become slaves to the magic. The shadowen are a perfect example of such antagonists. They can move from body to body, and they feed on the magic or life force of the Earth. They are addicted to it and when the energy of the Earth is released they are destroyed because they have come to rely on it so much that they can't exist without it. Magic is treated as an unstable force that can never really be mastered and is something best left to specific professionals who can handle it.
In reality I don't think that magic is an unstable force that warps people. If anything, I think that what leads to the abuse of magic is the instability a given person has in his/her life. Magic has a tendency to bring out best and worst of a person, because its various processes necessarily work with a person's internal values, beliefs, and perceptions. And if a person isn't adequately prepared (i.e. hasn't done a lot of internal work to sort those issues out), then the magic will draw out the dysfunction and let it run loose.
From my own observations, it seems that practical magic is reactive magic, done as a way to solve a problem or handle a crisis. The reason I consider it reactive is that rarely is the question asked: What is my role/responsibility for this problem? Far easier to blame the problem on others...until you notice that you have a string of similar problems that shows that the issue is partially derived from your own issues.
Effective magic is proactive magic and proactive magic involves doing a fair amount of internal work to address the issues that would otherwise come out in your life. I don't think its any coincidence that the need for me to do reactive problem solving via magic (or other methods) has significantly diminished thanks to doing internal work. If you solve the problem internally, you are less likely to get involved in the external trappings of the problem. But proactive magic is more than just doing internal work. Proactive magic is planning out how you want your life to manifest. It's lining everything up so that life becomes easier and more manageable.
A magician who is in touch with his/her internal reality is a magician who can take responsibility for how s/he contributes to the problems that s/he encounters. Such work can be hard, as it involves really knowing yourself, but the rewards make your life much easier to live and much more enjoyable in the long run.